In most operating conditions on a turbine engine, it is desirable to have every fuel nozzle flow an equal amount of fuel. Reducing nozzle to nozzle flow variation enables better control of local fuel-to-air ratios in the combustor and allows for uniform temperature distribution at the inlet to the turbine stage. Uniform fuel flow from nozzle to nozzle is accomplished by calibrating the nozzle's flow number (FN) at a key operating condition. The pressure drop of the nozzle is adjusted to be in a narrow range at a known flow. In most applications, the calibration point tends to be at a high flow, where the accuracy of flow from nozzle to nozzle is most critical.
Nozzle calibration is accomplished by adjusting the pressure drop of a flow restriction in the nozzle, which is typically in series with the tip restriction of the nozzle. This adjustable flow restriction in the nozzle is referred to as the calibration orifice. All nozzle flow typically passes through the calibration orifice and then the tip, which combine to give the nozzle its high-flow pressure drop characteristic.
By using the calibration orifice calibrated at high flow, the design challenge then becomes controlling flow of the fuel nozzle at low flow conditions. At low flow, the pressure drop in the nozzle can be very small and subject to variations caused by, e.g., check valves, head effect of the manifold, and other variations. Pressurizing valves (also known as metering valves) are used to keep small variations between nozzles from turning in to large percentage flow variations at low flows. The metering valve does this by controlling the relationship between pressure rise and nozzle flow rate; the flow number of the valve is small at lower flows and increases as flow increases. A functional schematic illustration of such a conventional fuel nozzle 101 having a metering valve 103 is shown in FIG. 5.
As will be recognized by those skilled in the art, the conventional metering valve 103 includes a valve spring 105, an inner spool 107, and an outer sleeve 109. Fuel enters the metering valve manifold 111 through ports 113. As the inlet fuel pressure exceeds the spring force acting on inner spool 107, the inner spool 107 begins to stroke to the right as shown in FIG. 5. As the inner spool 107 moves to the right in relation to outer sleeve 109, it opens metering port 115, which then allows fuel to flow through the orifice 117 and the tip restriction 119 and into the combustor.
In such conventional nozzles 101, the metering valve 103 is always in regulation. This means that it is always balancing the spring force of the valve against the pressure drop of the valve. Thus, such conventional metering valves 103 are always adding pressure drop to the nozzle 101, even at high flow, as may be seen from an examination of the graphical illustration of FIG. 6. In this graph, trace 121 illustrates the pressure drop across metering port 115, trace 123 illustrates the pressure drop across the calibration orifice 117 and the tip restriction 119, and trace 125 illustrates the percentage of total pressure drop across the fuel nozzle 101 that is allocated to the metering port 115. As may be seen, the pressure drop across the metering port 115 (trace 121) is fairly constant over the entire nozzle flow range, i.e. it continues to add pressure drop to the nozzle 101, even at high flows. While the pressure drop of the valve 103 is key to operation at low flow, it is not required at high flow. In fact, the additional pressure drop of the metering valve 103 (see trace 121) adds variation to the precisely calibrated performance of the nozzle 101, thus increasing the flow variation from nozzle to nozzle (compared to a nozzle with no valve) at high flow rates.
It is desired, therefore, for a pressurizing valve for use in a nozzle that is able to work normally (variable FN) at low flow, without adding additional variation to the critical calibrated high flow of the nozzle. The invention provides such a valve. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.